This invention relates in general to vehicular brake systems. More specifically, this invention relates to a hydraulic control unit having a self-priming pump for use in a vehicular brake system.
Conventional vehicular brake systems having anti-lock brake, traction control and/or vehicle stability control functions employ a hydraulic control unit. A typical hydraulic control unit used in these types of brake systems includes a housing having multiple fluid control valves or isolation valves for modulating the flow of fluid to a plurality of wheel brakes during designated braking events. The housing further includes multiple fluid control valves or dump valves for modulating hydraulic flow from the wheel brakes during designated braking events. During these designated braking events, a pump, disposed in the housing, transfers fluid from a master cylinder to the wheel brakes, and vice versa. The pump includes a pair of opposing pistons and sleeves. Each pair of pistons and sleeves defines a single variable-volume chamber. Each variable-volume chamber includes an inlet in fluid communication with the master cylinder and at least one of the dump valves. Each variable-volume chamber further includes an outlet in fluid communication with the master cylinder and at least one of the isolation valves.
Pump efficiency is defined by the flow rate of the fluid transferred from the variable-volume chambers divided by the flow rate capacity of the pump. The flow rate capacity of the pump is defined by the pump cycle rate multiplied by the combined volumes of the variable-volume chambers.
It is a noted concern that the pump efficiency is reduced when the temperature of the fluid decreases and/or when the pump cycle rate is increased. The reduction in the pump efficiency is significantly due to the drag placed on the fluid supplied to the variable-volume chambers, which in turn places a limit on the flow rate of the fluid supplied to the variable-volume chambers. The drag placed on the fluid supplied to the pump becomes more pronounced when additional components are placed in the fluid flow path of a given variable-volume chamber in advance of the inlet of the variable-volume chamber.
Accordingly, there is a need for a hydraulic control unit having a pump capable of operating efficiently at low ambient temperatures and/or at an increased pump rate.
It is also known that during braking events in which the pump draws fluid from the master cylinder while an operator is applying force to the brake pedal that the brake pedal may "shutter" or exhibit a stepped displacement. The stepped displacement of the brake pedal is a result of the pump displacing fluid from the master cylinder in an intermittent manner. In other words, the pump does not draw fluid from the master cylinder in a continuous manner, rather the pump only draws fluid from the master cylinder when moving from a top-dead-center position to a bottom-dead-center position. Consequently, the pump causes the brake pedal to shutter when the pump draws fluid from the master cylinder and the operator is applying force to the brake pedal.
Accordingly, there is a further need for a hydraulic control unit having a pump that reduces shuttering of a brake pedal associated with braking events in which the pump draws fluid from a master cylinder while an operator is applying force to the brake pedal.